Transient-current study of field-assisted emission from shallow levels in silicon

Transient currents due to the emission of carriers trapped on shallow doping levels are investigated in silicon junctions at low temperature. The current-vs-time curves present a peak which can be explained in terms of electric field activation. Simultaneous bulk resistivity measurements allow determination of a temperature range where the transient currents can be properly analyzed, i.e., between 20-25 K. The detrapping kinetics are then analyzed mathematically and the different possible electric field activation mechanisms reviewed: shallow-impurity impact ionization, Frenkel-Poole effect, tunneling, and phonon-assisted tunneling. Three-dimensional calculations are performed for these last two effects and tractable expressions are derived. In Si:P junctions, fairly good agreement between the theoretical model and experimental data is obtained for Frenkel-Poole effect only. In Si:B junctions, an electric-field-dependent peak is observed for ionization transient currents, but it is too broad to be fitted. This is shown to be due to the very high resistivity of our $p$-type samples in the temperature range investigated. Finally, the value obtained for the electron-capture cross section on phosphorous is discussed.